The present invention relates to apparatus for the coating of thin elongate elements, and in particular to the extrusion coating of optical fibers.
It is common to coat the exterior of thin elongate elements such as wire, optical fibers, etc., with a protective sheath. For example, optical glass fibers are coated with a polymer melt by passing the fiber through a die while flowing the melt onto the fiber. Conventional types of such coating dies are depicted in FIGS. 1 and 2 which illustrate a pressure coating die 10 and a sleeve coating die 12, respectively. In the pressure coating die 10, the fiber 14 is passed through a die tip 16 and then through an orifice 18 of a nozzle 20. The die tip 16 is spaced slightly rearwardly away from the nozzle orifice 18 so that polymer melt 22 can be fed through the nozzle orifice and into pressure contact with the fiber 14 as the latter exits the die tip 16. The fiber thus leaves the nozzle 20 closely coated with melt 22. The diameter of the nozzle orifice 18 corresponds to the desired outer diameter of the coating, so there is no need to draw-down the coating after leaving the die. The pressure coating technique is usually employed in cases where the polymer has no extensibility or melt strength, or where twisted bundles of wire or fibers are being coated so that the interstices are filled.
In the sleeve coating technique (FIG. 2), the discharge end 23 of the die tip 24 is flush with the discharge end 25 of the nozzle 26. The polymer melt 28 is applied in the form of a conical flow downstream of the nozzle 26 and is drawn-down to the desired outer diameter.
When converting a coating facility from one to the other of the pressure and sleeve techniques, it is necessary to exchange die assemblies, because the types of die assemblies depicted in FIGS. 1 and 2 are mutually incompatible. This results in the need to fabricate and keep on-hand multiple sets of die assemblies.
Also, in the event of a fiber breakage, it is difficult to rethread the fiber into the die tip of the types depicted in FIGS. 1 and 2 because the inlet end of the die tip through-bore is not readily accessible or visible in the usual mounting fixture, and/or because the downstream end of such through-bore may become blocked by polymer. As a result, it may be necessary to disassemble the die assembly in order to rethread the fiber.
It is, therefore, an object of the present invention to minimize or obviate problems of the type discussed above.
Another object is to enable a single die assembly to be adaptable to both pressure and sleeve coating techniques.
A further object is to provide a die assembly which facilitates the threading of the element to be coated.